Bench Box

This schematic is close to, but not exactly what is inside. For instance, I still have yet to put relay switches in. The switch is a simple rotary one. The main attenuator circuit is the right idea, but the final values were different. I probably did not use such a low value R_FB there, and the final values gave me attenuations of -0dB, -20dB, and -40dB. The GAIN control is an active Baxandall volume circuit from Douglas Self's Small Signal Audio Design. The idea is to get a good "linear-in-dB" rotation from a linear pot.

Again, not quite the final deal, but similar enough. The BOOST arrangement is also found in the AUX input amp that is just labelled "BOOST SWITCH" on the first schematic. I have yet to implement the input impedance selector switch. I haven't decided between mechanical switching or a separate input between hi and lo Z. Often I will test something that has a dc offset at power up that takes a long time to discharge with the hi Z input. Also, the hi Z probably adds noise. The hi Z is necessary for when testing high impedance outputs. The Echoplex, which has a very high output impedance coming off the plate of a triode, is a typical unit I need the hi Z input for.

The "Bench Box" is my multipurpose signal router that I have revised many times and it will always be changing. The basic idea remains the same.

The average device under test (DUT) will need a test signal at the input and I'll need to monitor the output. That is the most basic thing I do for every repair, so it better be quick and easy to do, right?

So my first concern in signal routing is managing all my test signals down to one 1/4" jack and managing all my monitoring gear to another 1/4" jack. For DUT jacks that aren't 1/4", I keep a variety of female 1/4" to RCA/XLR/ETC adapters handy. This way I can leave two 1/4" cables plugged into the bench box at all times and just attach adapters as needed.

SOURCE INPUTS

Guitar (Bench "test" guitar, passive Tele)

Sine Wave from Keithley (low dist, usually set to 440Hz)

Stereo -> Mono input to run test signals from the computer

Aux Panel Input (same amp design as return amp)

The Keithley and the computer stereo signal are set for +4dBu output.

The guitar input boosts the signal up to approximately +4dBu. There is only an internal trim pot to calibrate this. I never have need to adjust this however.

The stereo signal is buffered and then summed to mono.

The aux 1/4" input is high impedance and goes to an amplifier. The amp can be set for unity or variable boost in two steps.

Currently my Sweep Generator is hogging the aux input all day. Future revisions will give the sweep generator a rear input.

SOURCE AMPLIFIER

Mixer, Variable Gain and Attenuator Switch

The 3 known inputs are selected via a 3 pos rotary switch.

The Aux is summed with whatever is selected, so I mix 2 test signals if needed.

An attenuator switch selects -40dB, -20dB, or -0dB attenuation.

A variable gain goes from -inf to +20dB.

Power supply is +/-12V so the maximum voltage is limited by that. I plan on doing a +/-17V supply next time.

BYPASS SWITCH

This is setup like an effects pedal. There is a "true bypass" path that disconnects the 1/4" DUT jacks or activates the jacks and lights a LED to indicate "test" mode.

SEND JACK

Connected to scope CH 1

RETURN JACK

High impedance. Considering making the input impedance switchable.

Connected to Scope Selector Switch 1 and Return Amplifier input

RETURN AMPLIFIER

Attenuator Switch + Series gain

This amp drives:

Input to computer

Input to mixer

Peterson strobe tuner

(Comp and tuner have a shared attenuator knob)

SCOPE STUFF

CH 2 goes to scope selector switch, 3 position

1 = RETURN JACK/BYPASS SWITCH

2 = BNC panel jack with AC/DC and x10 switches

3 = Panel banana jack, fixed at x10 (for connection to speaker box)

Currently the 1/4" jack is wired to the RED tip jack. I never use this.

Black banana jack for ground connections.

MIDI jack. In the rear I have USB->MIDI with IN and OUT connections. In the front there is just 1 jack, and you select IN or OUT via a toggle switch. I only need to test one of the functions at a time and I just ran out of space for a second jack. Actually, I like the switch since I can use 1 cable for testing a DUT.